The properties of nematic or nematic-cholesteric liquid-crystalline materials are increasingly utilized for electro-optical display elements in order to effect significant changes in the optical properties of the latter, such as light absorption, light scattering, birefringence, reflectance or color, under the influence of electric fields. The functioning of display elements of this type is based, for example, on the phenomena of dynamic scattering, the deformation of aligned phases, the Schadt-Helfrich effect in the twisted cell or the cholesteric-nematic phase transition.
For the technical application of these effects to electronic components, liquid-crystalline dielectrics are required which must fulfill a large number of requirements. Chemical resistance to moisture, air and physical influences, such as heat, radiation in the infrared, visible and ultraviolet ranges and continuous and alternating electric fields, is of particular importance. Industrially usable liquid-crystalline dielectrics are also required to have a liquid-crystalline mesophase in the temperature range from at least +10.degree. C. to +50.degree. C., preferably from 0.degree. C. to 60.degree. C., and the lowest possible viscosity at room temperature, which preferably should not exceed 70.times.10.sup.-3 Pa.s. Finally, they must not have any characteristic absorption in the range of visible light, i.e., they must be colorless.
A number of liquid-crystalline compounds has already been disclosed, which fulfill the stability demands made for dielectrics intended for electronic components, and which are also colorless. These include, in particular, the p,p'-disubstituted phenyl benzoates described in German Offenlegungsschrift No. 2,139,628 and the p,p'disubstituted phenylcyclohexane derivatives described in German Offenlegungsschrift No. 2,636,684. In the two classes of compounds mentioned, and also in other known series of compounds with a liquid-crystalline mesophase, there are no individual compounds which form a liquid-crystalline nematic mesophase in the required temperature range from 10.degree. C. to 60.degree. C. As a rule, mixtures of two or more compounds are therefore prepared, in order to obtain substances which can be used as liquid-crystalline dielectrics. For this purpose, a compound having a low melting point and clear point is usually mixed with another compound having a markedly higher melting point and clear point. This normally gives a mixture, the melting point of which is below that of the lower melting component, while the clear point is between the clear points of the components. It is, however, not easy to prepare optimum dielectrics in this way, since the components having the high melting points and clear points frequently also impart a high viscosity to the mixtures. As a result, the switching times of the electro-optical display elements, produced with these mixtures, are extended in an undesirable manner.